Noise reduction methods for reducing the noise in a single audio input signal that may include voice or other desired signal components typically determine a gain function to apply in the frequency domain to a frequency domain representation of the audio input signal. Many methods are known for such gain determination. We call such gain determination input processing. Due to many factors, e.g., non-stationarity of the actual noise that is present, estimation errors in the methods, and the general requirement that such a method remove only undesired signals to isolate the desired signal, applying the gain determined by such input processing may produce various artifacts, such as speech distortion and musical noise. Musical noise artifacts may be caused by estimation errors in the frequency domain which lead to spurious peaks in the spectral representation of the signal after the determined gain function is applied. When a result is transformed back to the time domain, these peaks correspond to a tonal excitation whose frequency content may vary randomly and unnaturally from frame to frame.
An input processing method and system as used herein means a method and system that determines gains for noise reduction using a priori signal-to-noise (SNR) estimation or a method and system that determines gains not based on a priori SNR. For example, an input processing system that has multiple inputs can determine gains derived from spatial features. An input processing system that has one or more reference signals can determine gains that achieve echo suppression. Input processing also can determine gains to carry out one or more of perceptual domain-based leveling, perceptual domain-based dynamic range control, and perceptual domain-based dynamic equalization that take into account the variation in the perception of audio depending on the reproduction level of the audio signal, as described, for example, in commonly owned WO 2004111994 titled “METHOD, APPARATUS AND COMPUTER PROGRAM FOR CALCULATING AND ADJUSTING THE PERCEIVED LOUDNESS OF AN AUDIO SIGNAL.”
The outputs after application of such gains may be subject to musical noise or other types of artifacts.